1. Field of the Invention
This disclosure relates to a method of separating hydrogen from a hydrogen-containing stream using a hydrogen separation membrane system. More particularly, this disclosure relates to a method of separating a stream consisting essentially of hydrogen from impure hydrogen streams at temperatures greater than about 200° C. through the employment of a hydrogen separation membrane system.
2. Background of the Invention
Methods of producing hydrogen and carbon monoxide include steam reforming, gasification, or partial oxidation of natural gas, petroleum, coal, biomass, and municipal waste. Production of hydrogen from these sources is accompanied by production of carbon dioxide, carbon monoxide, and other gases. It is highly desired to separate hydrogen from these side-products and gaseous contaminants.
One currently used well-known and established method to separate H2 from impurities (i.e., other gases) is Pressure Swing Adsorption (“PSA”). PSA uses multiple beds, usually two or more, of solid adsorbent to separate impure H2 streams into a very pure (99.9%) high pressure product stream and a low pressure tail gas stream containing the impurities and some of the hydrogen. For example, synthesis gas (H2 and CO) may be introduced into one bed wherein everything but the hydrogen is adsorbed onto the adsorbent bed. Ideally, just before complete loading, this adsorbent bed is switched offline and a second adsorbent bed is placed online. The pressure on the loaded bed is subsequently reduced, which liberates the raffinate (in this case largely CO2) at low pressure. A percentage of the inlet hydrogen, typically 10 to 20 percent, is lost in the tail gas. Greater recovery means more CO2 and CO in the H2 product stream.
PSA is currently generally the first choice for steam reforming H2 plants because of its combination of high purity, modest cost, and ease of integration into the hydrogen plant. It is often used for purification of refinery off gases, where it competes with other kinds of membrane systems. One of the major disadvantages of this type system is that it operates at about 38° C. (100° F.). This entails a loss of thermal efficiency because the entire gas stream must be cooled prior to introduction into the pressure swing adsorption beds.
As the earth continues to warm, focus has been placed on effective methods for capturing and sequestering CO2 (a “greenhouse gas”) which has theoretically been linked to this warming trend. PSA is not advantageous from this standpoint, because the raffinate (i.e. tail gas) is produced at low pressure. Thus, if CO2 is to be captured from the system, large amounts of CO2 must be compressed from nearly atmospheric pressure to greater than 1000 psig. This compression from atmospheric pressure to 1000 psi can consume about 1/7th the total energy of the feed (e.g., coal). The critical pressure needed to liquefy CO2 for convenient transport and sequestration is 73.9 bar (1071 psi). Pressures well in excess of this (greater than 2000 psi) are required, however, to force the CO2 into oil wells or other underground storage sites.
Another disadvantage of PSA is that low raffinate pressure essentially limits the system to a single stage of water gas shift, WGS, which is often used to convert CO to CO2 and to produce additional hydrogen from synthesis gas streams. Limiting a hydrogen separation system to a single stage of WGS decreases the amount of CO conversion as well as the H2 recovery. PSAs are also undesirable compared to filters and membranes, due to mechanical complexity, which leads to higher capital and operating expenditures and, potentially, increased downtime.
Accordingly, an ongoing need exists for a process to separate hydrogen that does not require cooling of the feed stream prior to separation and decreases or eliminates compression requirements of separation products.